Abstract
In the bacterial flagellar motor, the cell-wall-anchored stator uses an electrochemical gradient across the cytoplasmic membrane to generate a turning force that is applied to the rotor connected to the flagellar filament. Existing theoretical concepts for the stator function are based on the assumption that it anchors around the rotor perimeter by binding to peptidoglycan (P). The existence of another anchoring region on the motor itself has been speculated upon, but is yet to be supported by binding studies. Due to the recent advances in electron cryotomography, evidence has emerged that polar flagellar motors contain substantial proteinaceous periplasmic structures next to the stator, without which the stator does not assemble and the motor does not function. These structures have a morphology of disks, as is the case with Vibrio spp., or a round cage, as is the case with Helicobacter pylori. It is now recognized that such additional periplasmic components are a common feature of polar flagellar motors, which sustain higher torque and greater swimming speeds compared to peritrichous bacteria such as Escherichia coli and Salmonella enterica. This review summarizes the data available on the structure, composition, and role of the periplasmic scaffold in polar bacterial flagellar motors and discusses the new paradigm for how such motors assemble and function.
Highlights
Reviewed by: Morgan Beeby, Imperial College London, United Kingdom Dipshikha Chakravortty, Indian Institute of Science (IISc), India
It is recognized that such additional periplasmic components are a common feature of polar flagellar motors, which sustain higher torque and greater swimming speeds compared to peritrichous bacteria such as Escherichia coli and Salmonella enterica
This review summarizes the data available on the structure, composition, and role of the periplasmic scaffold in polar bacterial flagellar motors and discusses the new paradigm for how such motors assemble and function
Summary
The flagellum (Figure 1) comprises the basal body, hook, and filament. The basal body functions as a rotary motor; the turning force (torque) generated by it is transmitted through the hook to the filament, causing it to spin (Zhao et al, 2014; Carroll and Liu, 2020; Takekawa et al, 2020). Four main types of flagellar arrangement have been observed: monotrichious bacteria (e.g., Vibrio cholerae) carry a single polar flagellum; amphitrichous cells (Campylobacter jejuni) have one or more flagella at both poles; lophotrichous bacteria (Helicobacter pylori) have multiple flagella at one pole; while peritrichous bacteria (Escherichia coli) possess multiple flagella distributed over the cell envelope (Schuhmacher et al, 2015). The flagellar motor is a remarkable nanoscale molecular engine that self-assembles in the cell wall from many protein components.
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